This invention relates to a DC to AC converter and more particularly to a ringing generator converting a DC input voltage to an AC output voltage with continuously adjustable frequency and amplitude on which a ringback tone may be superimposed.
In a typical DC to AC converter, a DC voltage from a DC source is supplied to a DC to DC converter which converts this input voltage to fixed or variable DC supply voltages. A signal generator or reference oscillator generates a reference signal which represents the desired output waveform. The fixed or variable DC voltages are supplied to a linear amplifier which is controlled by the reference oscillator. The linear amplifier generates an AC output voltage having a peak amplitude which is less than that of the fixed or variable DC voltages generated by the DC to DC converter.
The DC to AC converter described in Rhyne et al, U.S. Pat. No. 4,636,927 discloses an improvement in efficiency and a simplification of earlier DC to AC converters. U.S. Pat. No. 4,636,927 uses adjustable power supplies, i.e. "wraparound supplies," for the linear amplifier.
The wraparound supply of U.S. Pat. No. 4,636,927 is controlled by a low frequency reference oscillator which is also the reference signal from which the AC output is derived. The reference oscillator control of the wraparound supply causes it to follow the wave shape of the AC output from the linear amplifier, but at a slightly higher voltage magnitude. In this manner, the power dissipation in the linear amplifier is reduced, since the average voltage across the output power transistors is reduced from that with a fixed voltage DC power supply, which would have to be set for a voltage slightly above the peak AC output voltage of the linear amplifier.
The DC to AC converter of the Rhyne patent uses control means derived from the reference oscillator to control the supply voltages of the linear amplifier. The Rhyne control means achieve improved efficiency by reducing the average voltage across the power transistors in the linear amplifier from that obtained in earlier DC to AC converters that employ a constant DC supply. However, the Rhyne control means do not address operation of the linear amplifier under overload conditions.
Other prior devices include ringing generators. Some earlier ringing generators "clipped" the output voltage at the level where the maximum safe current was being supplied to the load, thereby causing distortion in the output voltage wave form.
Ringing generators have been connected in a parallel combination to increase output capacity. If redundancy is needed to increase the reliability of the parallel combination, the units are typically configured in a "2N" redundancy arrangement, where N is the number of units needed to drive the applied load. Two ringing generators, or two banks of ringing generators are connected to external monitoring and switch-over means.
The monitoring and switch-over means cause one bank of ringing generators to be connected to the load. Should this bank fail, the external monitoring and switch-over means disconnect it and connect the second bank of ringing generators to the load instead.